1. Field of the Invention
This invention relates to ultra-fine gold particles immobilized with oxides, methods for the production thereof, and oxidation catalysts, reduction catalysts, flammable gas sensor elements, and electrode catalysts having the ultra-fine gold particles immobilized with oxides as substantially the main component thereof.
2. Prior Art Statement
It has been known that ultra-fine gold powder having a particle diameter of smaller than about 0.1 .mu.m exhibits specific physical and chemical properties different from those of ordinary coarse gold powder ("Ultra-fine Powder", published by Agne Publishing Center, 1986).
Generally, any ultra-fine powder requires careful handling because it has large surface energy and great liability to coagulation. Particularly, an ultra-fine gold powder exhibits strong cohesiveness among the individual particles thereof and is liable to coagulate as compared with similar powders of other noble metals such as Pt and Pd.
In practical utilization, it is desirable to develop a method for immobilizing ultra-fine gold particles in a uniformly dispersed and deposited state on a carrier. For example, a method has been disclosed (Japanese Patent Public Disclosure SHO No. 60(1985)-238148) which produces a composite material having a gold compound dispersed in a metal oxide by the coprecipitation from a mixed aqueous solution containing water-soluble compounds of manganese, iron, cobalt, nickel, and copper and one of water soluble gold compounds. This method has a disadvantage in that it is incapable of immobilizing fine gold particles on a premolded catalyst carrier of a metal oxide or a molded support carrying thereon a metal oxide and it requires a great deal of gold.
There has also been disclosed another method which conducts deposition of ultra-fine gold particles on a carrier by immersing the carrier in an aqueous solution containing water soluble salts of gold and transition metals and urea and/or acetamide. (Japanese Patent Application No. SHO 60(1985)-192775: U.S. Pat. No. 4,698,324) This method has a disadvantage that it is incapable of accurately controlling the conditions for gold deposition and the deposition of gold in the aqueous solution requires as much as several hours' time. It has another disadvantage in that the gold component can be only partially deposited from the aqueous solution. This problem results in low efficiency in gold usage and therefore high production cost. The precipitates of gold obtained by this method tend to become heterogeneously dispersed and the control of the distribution of particle diameter is appreciably limited.